Handover is a vital part of any mobile communication system. A handover is the process of transferring an ongoing connection of a mobile station or user equipment (UE) from one base station (the serving) to another base station (the target) in order to accomplish a transparent service over a larger area. The handover should take place without any loss of data and with as small an interruption as possible.
To enable a handover, it is necessary to find a suitable target cell, and to ensure that it is possible to sustain reliable communication with that target cell. Candidates for suitable target cell are usually stored in so-called neighbour lists, which are stored at least at the serving base station. To make sure that it is possible to sustain reliable communication with the target cell, the connection quality in the target cell needs to be estimated before the handover can take place.
The quality in the target cell is estimated by measurements related to the mobile station. Both downlink or uplink measurements can be considered. Relying solely on uplink measurements can be quite dangerous, since the uplink connection quality can be different from the downlink connection quality. Therefore, handovers in cellular systems are typically based on downlink measurements.
In legacy systems, i.e. in known systems, all base stations continuously transmit pilot signals that mobile stations in neighbour cells use to estimate the target cell quality. This is true in Global System for Mobile Communications (GSM) where a pilot signal is transmitted on a Broadcast Control CHannel (BCCH), Wideband Code Division Multiple Access (WCDMA) where a pilot signal is transmitted on a Common Pilot CHannel (CPICH) and in Long Term Evolution (LTE) where a pilot signal is transmitted as a Cell Specific Reference Signal (CRS). This leads to that it is possible to estimate the quality of neighbour cells with relatively good accuracy.
When handovers are based on downlink measurements, the mobile station needs to synchronize to transmission from the new (target) base station so that it can measure and decode the signal from the target base station. As most cellular systems are non-synchronized, this process may take a rather long time, and it also puts requirements on the design of the signals transmitted from the target cell. In legacy systems, the continuous pilots are used for this.
Synchronizing to the target cell can take a long time. The process typically involves correlating a known pilot signal with different time-shifted replicas of the received signal. Without any prior knowledge of what time-shifts are probable, this may take a long time, increase the risk of false correlation peaks, and imply extensive receiver processing at the mobile station.
Furthermore, in future mobile systems, there may not be any pilot signals that are continuously transmitted from all base stations. Instead, these pilots may be switched on only when they are necessary. This will make the synchronization task even more challenging, since a missed measurement session due to an erroneous timing estimate implies additional control signalling to initiate another session and a risk for radio link failure due to serving cell signal deterioration.
There is thus a need for a mobile station mobility measurement procedure that limits the timing uncertainty when performing mobility measurements on target cells in non-synchronized networks.